Making secret messages and IR t-shirts

Here's a trick. Take a sheet of Congo Blue filter and overlay it with Primary Red. It looks black to your unaided eyes. Now wear your IR filter goggles and observe those "black" filter sheets under incandescent light (or take them outdoors into sunlight.) You'll find that the sheets of Congo Blue plus Primary Red now appear to be transparent! They look a bit grey rather than totally clear, so you might want to try using a different colored filter instead, for example use a couple of layers of Roscolux #385 "Royal Blue" Unaided eyes think the red/blue filter stack is totally black, but your IR goggles let you SEE RIGHT THROUGH the filter sheets. Write a secret message on a piece of paper and cover it up with the "black" IR filter stack. Normal humans will see nothing but a shiney black square. But with your goggles you can see the secret message. Make IR-only signs. "No Cyborgs beyond this point." "Human infants taste terrible!" "Chlorine-breathing reptoids out of US Congress!" Any eyes that possess, ahem, enhanced longwave response will see your hidden message, but all of the "normals" will just see a featureless black square.

Here's another way to do the same thing. First use the goggles to look at different kinds of dark clothing. Find some cloth that looks light grey in the infrared, but looks dark black when you take the goggles off. It's easy to make a secret message with this cloth. Just write on that black cloth using black magic marker. Human eyes can't see the black-on-black. But if you wear the goggles on a sunny day, then the black writing will be clearly visible against the light cloth. (Most black magic markers have ink which is black in both the visible and the IR.) You can draw anything you want to on your black clothing. Only people with IR goggles (or IR cameras) can see it. [NOTE: I made some signs like this, and I found that I can still see the lettering by eye if large block letters are used. The large regions of "sharpie marker" ink is still visible on the black cloth. Run the black cloth through the wash to reduce this problem. On the other hand, thin writing is still invisible. It's only the big black blocks that can be seen by humans if they're paying attention.]

Test for better filters

Here's a trick that demonstrates that you're really dealing with IR. If you have a "swatch pack" of Lee color filters, find the Congo Blue #181 and the Peacock Blue #115. To normal human eyes, Congo blue appears almost opaque black, and Peacock blue looks transparent sky-blue. Now wear your IR filter goggles and look again. (You'll need sunlight or an incandescent bulb for illumination.) You'll find that the Congo Blue filter is no longer opaque! It now looks transparent... but now you can't see through the Peacock Blue. In the IR band, their roles are reversed. The Peacock blue filter is a black absorber under IR light, while the Congo Blue is transparent. Look at other filters in the Lee filter swatch-pack. You're really seeing the IR transmission of these filters, and as with Peacock Blue, the ones which look black in the near-IR are often very transparent in the visible.

Goth-ray Vision

Remember that I mentioned that certain clothing looks black for human eyes, but looks white in the IR? Certain dark blue dyes act this way. Some new blue-jeans look white in the infrared, while black work-pants appear black, but in the visible spectrum they both look very dark. Find yourself a black windbreaker which appears white in the infrared. Use carbon-based ink to put a nasty message or some disturbing artwork on the back, and only IR cameras (and IR filter goggles) will see it. Do "Goth Warchalking", where you write on bluish-black paper with black magic markers, and the resulting messages are only visible to these weirdos who go around wearing black-lensed mad scientist goggles.

Now I need to find some IR-absorbing spray paint and magic markers. I want to do the opposite to the above. I want some kind of paint which looks totally water-clear to human eyes, but looks totally black at 720nM infrared. Why? Because then I can put IR graffiti all over everything, and nobody can see it unless they're equipped with IR goggles. I'll draw "crop circles" on lawns and city streets that only IR cameras can see! Maybe get some huge nasty tattoos on my face which are invisible to mundane eyes. Hmmmm, I wonder if anyone is already doing this. If I keep a lookout while wearing goggles maybe I'll find secret messages on city sidewalks written by the MIBs. Search google on "infrared paint."

A view more Infra-reddly

Congo blue filers give your eyes a peak sensitivity of around 720nM. That's definitely into the IR band which starts at 700. If you want your vision to be much deeper into the IR, you can use a different Lee or Rosco filter, one with an even deeper IR cutoff. One such filter is Lee #120 "Deep Blue." This filter passes much more blue light than Congo blue, so you'll need to use three or four layers of Deep Blue, plus two or three layers of Primary Red.

The result is different than the congo blue goggles. With these goggles you can barely see anything at all, even in brightest daylight. But after about 15 seconds your eyes grow used to the dark. And then the sky looks far more black, and the plants and trees are even whiter. Humans are boring: they're all just grey-red, including clothing and hair. But human faces are weird because everyone's eyes look huge and dark.

Ditch the goggles, make an IR floodlight

In a dark room or during a moonless night these goggles are worthless. Their whole purpose is to block the background light from the environment, and if there IS no background light, then you don't need any goggles to see a bit of IR. So, if you want to experiment with direct viewing of IR LEDs or (dangerous!) IR diode lasers, just go into a well-darkened room and observe IR sources directly. But that leads to another idea: don't put filters on your *eyes*, instead put the congo blue layers over a white light source. If you have a simple theatrical floodlight that blocks any spill from the rear, and can take a colored filter in the front, then you can make a high power near-IR floodlamp. Give it a few layers of Congo Blue and one or two sheets of Primary Red to cut out the blue leakage. This is NOT the same as an 850nM LED floodlamp used with security cameras. In a dark room it looks fairly strange; appearing as dim red light until you aim it at a human face and find that their skin is translucent, their hair is wispy grey, and their eyes are alien-looking black. As usual, certain types of black cloth instead look grey (so your black Sharpie-marker artwork suddenly becomes visible.) Also, a sheet of congo blue looks nearly transparent when held in your hand. If people wear the IR goggles, the filters don't look very dark, and you can see their eyes. And psychologically its very eerie, since these effects are occuring, yet you're not wearing any goggles on your face. It might be pretty cool if used to light an "infrared art gallery" with black-on-black velvet paintings. Or if used to create incredibly intense 900nM illumination (and if this doesn't damage human eyes,) then spandex clothing worn in the gallery would appear transparent.

Speaking of art, here's an idea that requires a bright outdoor environment (such as Burning Man.) Build a booth out of transparent plastic. Cover the entire thing with layers of Congo Blue and Red. Make sure the door gives a good light-seal. Perhaps add a ventilation fan, since it'll get hot in there. Now climb inside, get used to the dark, and look around. The entire world will look like "IR goggles-view!" But that's just the first part. Now build one or two more of these booths and place them about ten feet apart. The outside observers see black shiny monolith booths, but a person inside a booths think the *other* booths are nearly transparent. Wave to the people in the other booths. Only they can see you, yet you might be surrounded by a clueless crowd outside the booths. It's almost like being invisible. Now do other things that might spring to mind. Go wild. But remember: I'M WEARING IR GOGGLES, so the "opaque" booths are transparent to me as well.

How do they work?